Note: Descriptions are shown in the official language in which they were submitted.
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WO 98/58093 PCT/SE98/01189
STAINLESS STEEL POWDER
The present invention concerns a stainless steel
powder and a method of producing this powder. The powder
according to the invention is based on a water-atomised
stainless steel powder and has improved compressibility.
Components prepared from this powder have improved
mechanical properties.
The atomisation process is the most common tech-
nique for fabricating metal powders. Atomisation can be
defined as the break-up of a liquid (superheated) metal
stream into fine droplets and their subsequent freezing
into solid particles, typically smaller than 150 gm.
Water atomisation gained commercial importance in
the 1950's when it was applied to the production of iron
and stainless steels powders. Today, water atomisation
is the dominant technique for high-volume, low-cost
metal powder production. The main reasons for using the
technique are low production costs, good green strength
due to irregular powder shape, microcrystalline struc-
ture, high degree of supersaturation, the possibility of
forming metastable phases, no macrosegregation and that
the particle microstructure and shape can be controlled
by the atomisation variables.
During the water atomisation process a vertical
stream of liquid metal is disintegrated by the cross-
fire of high pressure water jets. The liquid metal drop-
lets solidify within a fraction of a second and are col-
lected at the bottom of the atomising tank. The tank is
often purged with an inert gas, such as nitrogen or ar-
gon, to minimise the oxidation of the powder surfaces.
After dewatering the powders are dried and in some cases
annealed, whereby the surface oxides formed are at least
partly reduced. The main disadvantage with water atomi-
sation is the powder surface oxidation. This disadvan-
tage is even more pronounced when the powder contains
CONFlRMAt10N COPY
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easily oxidisable elements such as Cr, Mn, V, Nb, B, Si,
etc.
Because of the fact that the possibilities of sub-
sequent refining of water-atomised powders are very lim-
ited, the conventional way of producing stainless mate-
rial (% Cr > 12%) from a water-atomised steel powder
usually requires very pure and accordingly very expen-
sive raw materials e.g. pure scrap or selected scrap. A
frequently used raw material for the addition of chro-
mium is ferrochrome (ferrochromium), which is available
in different qualities containing different amounts of
carbon, the qualities containing least carbon being the
most expensive. As it is often required that the carbon
content of the final powder should not exceed 0.03% the
most expensive ferrochrome quality or selected scrap has
to be chosen.
In addition to the water atomisation method it is
possible to subject a metal melt to gas atomisation.
This method is, however, practised for special purposes
and it is rarely used for the production of steel pow-
ders to be sintered or sinter-forged, which is the major
application in the field of powder metallurgy techno-
logy. Furthermore, gas atomised powders require hot
isostatic pressing (HIP), a reason why components pro-
duced from this type of powders are very expensive.
In the oil atomisation process for producing steel
powders oil is used as the atomising agent. This process
is superior to water atomisation in that the oxidation
of the steel powder does not occur, i.e. the oxidation
of alloying elements dc:_~ not occur. However, carburisa-
tion of the resulting powder i.e. diffusion of carbon
from the oil to the powder occurs during atomisation,
and decarburisation has to be carried out at a succeed-
ing step. The oil atomisation process is also less
acceptable than the water atomisation process from an
environmental point of view. A process for producing a
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low-oxygen, low-carbon alloy steel powder from an oil
atomised powder is disclosed in the US patent 4 448 746.
It has now unexpectedly been found that stainless
steel powders can be obtained from a water-atomised pow-
der from a wide varietv of inexpensive raw materials,
such as carburized ferrochrome, over-reffined ferrochrome,
pig iron etc.
In comparison with conventionally produced stain-
less steel powders based on water-atomisation the new
powder has a much lower impurity content, especially
with respect to oxygen and to some extent sulphur after
sintering. The low oxygen content gives the powder a
metallic gloss instead of the brown green colour, which
distinguishes a conventional water-atomised stainless
steel powder. Furthermore, the density of green bodies
prepared from the new powder is much higher than the
density of green bodies prepared from conventional
water-atomised powders. Important properties, such as
tensile strength and elongation, of the final sintered
components prepared from the new powders are as good or
even better when the new powders according to the p-
-resent invention are used. Another advantage is that the
sintering process can be carried out at lower tempera-
tures than today's common practice, a reason why the
selection of furnaces will increase. Additionally the
energy consumption will be reduced both as a result of
the lower sintering temperature and of the lower tem-
perature needed for the melting of the raw materials for
the water-atomisation. Another consequence of the lower
melting temperature is that the wear on the furnace
lining and atomising nozzles can be reduced. An impor-
tant advantage is also as indicated above that less ex-
pensive chromium containing raw materials can be used.
The number of chromium containing raw materials can also
be increased.
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The US Patent 3 966 454 concerns a--rocess in which
carbon is added to an iron melt before water-atomising
and the water-atomised powder is subsequently subjected
to induction heating. This known process is not con-
cerned with the problems encountered in the
manufacturing of stainless steel products distinguished
by a high chromium content and low oxygen and carbon
contents.
A critical feature of the invention is that, during
the water-atomisation process, the carbon content of the
metal melt is adjusted to a value which is decided by
the expected oxygen content after the atomisation pro-
cess. The expected oxygen content after the atomisation
is decided either empirically or by taking a sample of
the melt before the atomisation. Normally the oxygen
content of a metal melt containing common raw materials
for steel production varies between 0.4 and 1.0 % by
weight of the melt. The carbon content of the melt is
then adjusted until an oxygen:carbon weight ratio of
about 1.0 -3.0 is obtained. Usually carbon has to be
added to the melt and the addition could involve addi-
tion of graphite. Alternatively more carbon containing
raw materials could be selected. The carbon content of
the molten steel as well as of the new water-atomised
powder should vary between 0.2 and 0.7, preferably
between about 0.4 and about 0.6 % by weight. Naturally
and if required the amount of carbon can be fine
adjusted by adding minor amounts of carbon, such as
graphite also after the water-atomisation.
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4a
According to one aspect of the present invention,
there is provided a process for producing an annealed water
atomised stainless steel powder consisting of, by percent of
weight:
a)
10-30% Cr
0-5% Mo
0-15% Ni
0-1.5% Si
0-1.5% Mn
0-2% Nb
0-2% Ti
0-2% V
b)
not more than 0.2% 0
not more than 0.05% C
c) and not more than 0.5% of impurities, the
balance being iron;
wherein the process comprises:
- preparing a molten steel of the above defined
components a) and c) while the carbon content is adjusted
within the range of 0.2-0.7% by weight to a value which
results in an oxygen/carbon weight ratio 1-3 in the melt and
in the resulting powder after water atomising;
- water atomising of the melt to produce the
atomised powder; and
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4b
- annealing the atomised powder at a temperature
of at least 1120 C by heating in a furnace in order to
obtain the annealed powder having not more than 0.2% 0 and
not more than 0.05% C.
In order to obtain a powder having the
advantageous properties mentioned above the obtained carbon
containing water-atomised powder is subjected to an
annealing step at a temperature of at least 1120 C,
preferably at least 1160 C. The process is preferably
carried out in a reducing atmosphere under controlled
addition of water, but could also be carried in any inert
atmosphere such
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as nitrogen, or in vacuum. I'he upper 'imit =or rhe
annealing ..empera'.ure _s about 1260 C. Depending cn the
selected temperature the anneal'ng Lime raay vary between
m.inutes and a few hou-s. A normai annealing time is
5 about 15 to 40 minutes. The annealing can be carried out
continuously or batch-wise in furnaces based on
conventional heating, such as radiation, convection,
conduction or combinations thereof. Er_amoles of furnaces
suitable for the annealing process are belt furnaces,
rotary heart furnaces, chamber furnaces or box furnaces.
The amount of water required for reducing the car-
bon can be calculated based on measurements of the con-
centration of at least orie of the carbon oxides formed
during the annealing step e.g. as disclosed in
International Publication No. WO 98/03291. Preferably the
water is added in the form of moist H2 gas or steam.
The most preferred embodiment of the invention con-
cerns the preparation of an annealed, water-atom7sed
powder, which has a chromium content of at least 10 %,
an oxygen content below 0.2, preferably below 0.15 and a
carbon content lower than 0.05, preferably below 0.03
and most preferably below 0.015 % by weight.
Preferably the annealed powder as well as the
water-atomised powder according to the invention could
include, by percent of weight, 10-30 % of chromium,
0-5 % of molybdenum, 0-15 % of nickel, 0-1.5 % of sili-
con, 0-1.5 % of manganese, 0-2 0 of niobium, 0-2 % of
titanium, 0-2 o of vanadium and at most 0.3 % of inevi-
table impurities and most preferably 10-20 % of chro-
mium, 0-3 % of molybdenum, 0.1-0.3 % of silicon, 0.1-0.4
% of manganese, 0-0.5 % of niobium, 0-0.5 of titanium,
0-0.5 % of vanadium and essentiallv no nickel or alter-
natively 7-10 % of nickel.
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The invention is further illustrated by the follow-
ing non limiting example:
Two raw powders, grade 410 and grade 434 were
prepared from ferrous raw material consisting of
ferrochrome carbure having a carbon content of 5 % by
weight and a low carbon stainless scrap. The ferrous raw
materials were charged in an electric charge furnace in
amounts adjusted to give at most 0.4 % of carbon in the
steel powder after water atomising. After melting and
water atomising the two raw powders, grade 410* and
grade 434*, had the composition given in the following
table 1.
TABLE 1
Grade % Cr % Mo % Si o Mn % C % 0-tot
410* 11.5 0.10 0.11 0.34 0.41
434* 17.6 1.0 0.14 0.1 0.37 0.48
*Water atomised carbon containing steel powder according
to the invention
The powders were then annealed at a temperature of
1200 C in a belt furnace having an atmosphere
essentially consisting of hydrogen gas. Moist hydrogen
gas i.e. hydrogen gas saturated with H20 at ambient
temperature, and dry hydrogen gas, were introduced into
the heating zone. The amount of ~.=toist hydrogen gas was
adjusted with an IR probe inte d for CO measurement.
An optimal reduction of the ox; n and carbon could be
obtained by using this probe and an oxygen sensor.
In the Table 2 below the compositions of the pow-
ders according Table 1 after the annealing process
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according to the present invention are disclosed as
powder 410** and 434** respectively.
TABLE 2
Grade % Cr %Ni %Mo %Si %Mn %C %O %N
410** 11.5 0.10 0.11 0.005 0.079 0.0004
410ref 11.9 0.15 0.76 0.15 0.007 0.23 0.03
434** 17.6 1.0 0.14 0.1 0.01 0.079 0.0009
1434ref 16.8 1.0 0.8 0.16 0.01 0.30 0.05
The powders 410ref and 434ref are conventional
powders, which are commercially available from
Coldstream, Belgium, which powders have only been
atomised but not annealed according to the present
invention.
The tables 1 and 2 disclose that particularly the
oxygen content is dramatically reduced during the
annealing process according to the invention. Also the
influence on the nitrogen content is positive.
From the following Table 3 it can be seen that the
annealed powder according to the present invention con-
tains less slag particles than the conventional powders.
TABLE 3
AD Flow Sieve analy- B.E.T Non metallic inclu-
sis sions (number/cm)
Mate- g/cro s/50g <45 m <150Eun m /kg +50- +100- +200N.m
rial 100 m 200 m
410 2.95 28.2 28.0 0.4 80 57.1 3.1 -
ref
410** 3,03 26.3 11.3 17.0 45 1.2 -
434 2.78 29.7 27.5 0.2 85 76.5 3.9 -
ref
434** 3.16 24.9 9.3 18.5 50 2.9 - -
TABLE 4
0
Material Sintered Dimen- Hardness Tensile Yield Elonga- Transverse 00
density sional HV 10 strength stress tion Rupture o
change (o) (MPa) (MPa) (o) Strength
(MPa)
1200 H2 410 Ref. 6.80 -1.61 82 253 157 11.09
410 ** 6.90 -1.07 70 238 126 21.14
434 Ref. 6.60 -1.81 64 236 192 4.99
434 ** 6.74 -1.06 74 267 175 15.01
1200 D.A. 410 Ref. 6.57 -0.30 278 584.2 >
410 ** 6.74 -0.09 287 528.4
43 ; Ref. 6.54 -1,43 227 291 195 2.34 592.3
434 ** 6.72 -0.82 273 496 350 0.87 862.1
00
1120 H2 410 Ref. 6.57 -0.43 80 131 111 1.43
410 ** 6.78 -0.41 68 239 119 10.71
434 Ref. 6.38 -0.63 66 148 134 1.46
434 ** 6.65 -0.52 73 249 165 12.05
1120 D.A. 410 Ref. 6.49 0.04 258 246.8
410 ** 6.72 0.02 291 377 - 0.05 631.8
434 Ref. 6.22 0.28 260 245.7
434 ** 6.63 -0.17 238 329 236 0.92 665.1
** = Sintered products prepared by using the water atomised and annealed
powder according to the present invention.
Ref. = Conventional material ..~
00
~.+
-+
OC
ID
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The above table 4 discloses the mechanical
properties of the materials after sintering in hydrogen
(H2) and dissociated ammonia (D.A.).
Table 5 discloses the green density, the green
strength and the springback.
TABLE 5
Material Green density Green strength Springback
(g/cm3) (MPa) M
410 ref 6.60 11.4 0.14
410** 6.77 11.3 0.13
434 ref 6.39 13.1 0.16
1434** 6.63 6.5 0.11
It can be concluded that the annealed 410** powder
according to the invention has a fines content (-45pm)
i.e. about 10 % compared with 30-35 % for the conven-
tional grades 410ref. The oxygen content is much lower
i e less than 0.10 % compared with 0.20 - 0.30 %. The
number of inclusions are surprisingly low. The green
density is increased with approximately 0.25 -0.50 for
both 410** and 434**. The sintered density is increased
with approximately 0.25-0.35 %. The oxygen pick up
during sintering is much lower for the powder according
to the present invention. Finally it could be observed
that the powder particles according to the invention
exhibited a more metallic brightness.
